Ca2+-stimulated adenylyl cyclases regulate the L-type Ca2+current in guinea-pig atrial myocytes

Ca2+-stimulated adenylyl cyclases (ACs) have recently been shown to play important roles in pacemaking in the sino-atrial node. Here we present evidence that Ca2+-stimulated ACs are functionally active in guinea-pig atrial myocytes. Basal activity of an AC in isolated atrial myocytes was demonstrated by the observations that MDL 12,330A (10 mu M), an AC inhibitor, reduced L-type Ca2+ current (ICaL) amplitude, while inhibition of phosphodiesterases with IBMX (100 mu M) increased ICaL amplitude. Buffering of cytosolic Ca2+ by exposure of myocytes to BAPTA-AM (5 mu M) reduced ICaL amplitude, as did inhibition of Ca2+ release from the sarcoplasmic reticulum with ryanodine (2 mu M) and thapsigargin (1 mu M). [ Ca2+] i-activated calmodulin kinase II (CaMKII) inhibition with KN-93 (1 mu M) reduced ICaL, but subsequent application of BAPTA-AM further reduced ICaL. This effect of BAPTA-AM, in the presence of CaMKII inhibition, demonstrates that there is an additional Ca2+-modulated pathway (not dependent on CaMKII) that regulates ICaL in atrial myocytes. The effects of BAPTA could be reversed by forskolin (10 mu M), a direct stimulator of all AC isoforms, which would restore cAMP levels. In the presence of BAPTA-AM, the actions of IBMX were reduced. In addition, inclusion of cAMP in the patch electrode in the whole-cell configuration prevented the effects of BAPTA. These effects are all consistent with a role for Ca2+-stimulated AC in the regulation of atrial myocyte ICaL.